The present invention relates to a process for the preparation of a master batch comprising an organic peroxide and a polymer capable of being cross-linked with the peroxide.
The need for preparing master batches of peroxides arises from the fact that the peroxides that are commonly used for, for instance, the cross-linking of polymers are liquid or have such a low melting point that they melt upon being mixed into the polymer as a result of the frictional heat then developed or because of the mixing taking place at elevated temperature. This complicates intermixing in that the peroxide flows away and/or lubricates the polymer, as a result of which the polymer particles no longer stick together during mixing and escape the mixing action. In order that the peroxides may yet be properly dispersed in the polymer, generally in a weight ratio of 0.5-5.0%, relatively long mixing times are required. The high temperatures that occur then may give rise to premature cross-linking of the polymer and promote the development of flammable or explosive peroxide vapors.
To avoid the aforementioned difficulties it has been proposed to incorporate peroxides into solid carriers such as silica, chalk and talc. Although in such a manner the peroxides may be incorporated into the polymer more rapidly, it has been found that the carriers are difficult to disperse rapidly and uniformly in the polymer. For some uses the presence of such carriers in the final product is undesirable.
The foregoing problems have led to the development of master batches. Characteristic of master batches is that the concentration of the peroxide in the polymer is considerably higher than the concentration in which the peroxide is to be mixed into the polymer to be finally cross-linked. The polymer from which the master batch is built up should be identical with or closely related to the polymer to be finally cross-linked. It has been found that when these master batches are mixed in, uniform and rapid dispersion of the peroxide is obtained. The master batches known so far are generally prepared with the aid of roll mills, internal mixers or compounding extruders. Such methods have the disadvantage, however, that mixing the peroxide into the polymer is attended with the release of heat or the need for using heat, which may result in partial decomposition of the peroxide.
Internal mixers and compounding extruders are entirely closed during mixing, so that decomposition of the peroxide induced by overheating or contamination may be attended with a pressure build-up. Master batches mixed on a roll mill moreover have the disadvantage that they contain a relatively high proportion of filler and are therefore relatively hard. This is to be attributed to the fact that in order that the peroxide may be properly mixed in, it must be incorporated into a solid carrier. Furthermore, the preparation of such master batches is very labor intensive and the resulting product is obtained in the form of sheets. To facilitate handling of the master batches and in view of the envisaged application they are preferably chopped or cut into granules.
It should be noted that U.S. Pat. No. 3,301,813 describes a process for the preparation of a powdered concentrate consisting of 40-60% by weight of a liquid additive and 60-40% by weight of a dry, porous polymer. In said patent specification no mention is made of peroxides. Concentrations of particular additives in thermoplastic polymers are also described in U.S. Pat. No. 3,637,571. In that case, however, the polymer is contacted with solutions or suspensions of the additive and peroxides are not mentioned in the specification either. Moreover, the method described is unsuitable to be used for the preparation of master batches of peroxides because the required high concentration of peroxides cannot be attained, at least not within an acceptable contact time.
In Netherlands Pat. No. 7,313,256, there is revealed a process of the type described in U.S. Pat. No. 3,637,571, for incorporating relatively small amounts of peroxides into the polymer to be finally cross-linked. As can be derived from the degree of cross-linking mentioned in Example 4 of Netherlands Pat. No. 7,313,256, only 2-3% by weight of peroxide is incorporated into the polymer following its being contacted with a 40%-solution of dicumyl peroxide for 75 minutes at 65.degree. C.
West German Pat. No. 2,124,941, too, describes a process for incorporating peroxide, in an amount of 0.5-5% by weight, into the polymer which is finally to be cross-linked. Apart from the fact that the method proposed is unpractical for the preparation of master batches, the other methods described would give rise to excessive sticking together of polymer particles.
Finally, British Pat. No. 1,489,580 describes a process in which shaped polymeric objects are contacted with a stable peroxide emulsion. The method is only suitable for incorporating relatively small amounts of peroxide and can therefore not be used for the preparation of master batches.